A solar cell is a generator directly converting sunlight to electric energy. The solar cell is roughly divided into a cell using a silicon semiconductor as a material and a cell using a compound semiconductor as a material. The silicon semiconductor solar cell includes a single crystal silicon solar cell, a polycrystalline silicon solar cell and an amorphous silicon solar cell.
The compound semiconductor is a semiconductor produced by combining a plurality of elements. The compound semiconductor cell includes a solar cell using a group III-V compound semiconductor (for example, GaAs) composed of a combination of a group III element such as Al, Ga or In and a group V element such as As or Sb, and a solar cell using a group II-VI compound semiconductor (for example, CdS or CdTe) composed of a combination of a group II element such as Zn or Cd and a group VI element such as S, Se or Te. In addition, development of a copper indium selenide solar cell, a dye-sensitized solar cell, an organic thin-film solar cell, etc. is also advanced.
A typical solar cell module is formed from a surface protecting material, a sealing material, solar cells, a back surface protecting material and a frame. As illustrated in FIG. 7, the main components of the solar cell module 71 include a surface protecting material 72, a sealing material 73, solar cells 74 and a back surface protecting material 76. A plurality of the solar cells 74 is connected in series by wirings 75 to form the solar cell module. A frame (not illustrated) is arranged at end portions or peripheral edge portions of the solar cell module.
As the surface protecting material 72, is used, for example, a reinforced glass plate, a transparent plastic plate or a transparent plastic film. As the sealing material 73, is commonly used an ethylene-vinyl acetate copolymer. As the back surface protecting material 76, is used, for example, a single-layer or multi-layer plastic film, a plastic plate, a reinforced glass plate or a metal plate (for example, aluminum plate or painted steep plate). As the frame, is commonly used, for example, aluminum light in weight and excellent in environmental resistance.
The structure of the solar cell 74 varies according to the kind of the solar cell used. For example, a structure that n-type silicon and p-type silicon and n-type silicon are joined, and electrodes are respectively arranged therein is representative of a silicon semiconductor solar cell. Examples of other solar cells include those having a layer structure of “collecting electrode/transparent conductor layer/photo-activated semiconductor layer/reflecting layer/conductive substrate”. The photo-activated semiconductor layer is formed of, for example, an amorphous silicon semiconductor. That obtained by arranging and connecting a plurality of solar cells and packaging them with a surface protecting material, a sealing material and a back surface protective material is called a solar cell module. That obtained by connecting a plurality of solar cell modules is called a solar cell array.
The solar cell module (including the array) is generally placed in the outdoors, and its operation state is thereafter retained over a long period of time. In order to operate the solar cell module over a long period of time in the outdoors, it is necessary for such a module to have excellent durability under a severe environment. Therefore, the surface protecting material, sealing material and back surface protecting material (hereinafter referred to as “backsheet”) of the solar cell module are required to have a function of protecting the solar cells over a long period of time under a severe natural environment surrounding the solar cell module.
In the backsheet for solar cell module, its surface (outermost surface) on the side opposing the solar cells is exposed directly to the outdoors. A surface (surface adjoining the sealing material) of the backsheet for solar cell module on the side of the solar cells is exposed to sunlight passing through between respective solar cells or between respective solar cell modules. Therefore, the backsheet for solar cell is required to be excellent in various properties such as light resistance, weather resistance, heat resistance, moisture resistance, water vapor barrier property, electrical insulating property, withstand voltage, mechanical properties, chemical resistance, salt resistance, stain resistance and adhesion property to sealing materials.
The backsheet for solar cell module is also required to be beautiful in the appearance of the surface on the side of the solar cells and additionally to have a function of efficiently reflecting sunlight incident on the backsheet in addition to the above excellent various properties. If the incident light passed through between the respective solar cells can be efficiently reflected by the backsheet, the power conversion efficiency of the solar cells is improved by the reflected light.
Japanese Patent Application Laid-Open No. 2002-100788 (Patent Literature 1) discloses backsheets for solar cell covering material, which are respectively composed of a 3-layer laminate of a hydrolysis-resistant resin film, a metal oxide-coated film and a white resin film; and a 2-layer laminate of a hydrolysis-resistant resin film coated with a metal oxide and a white resin film.
Patent Literature 1 describes that the white resin film is arranged on the innermost layer of the backsheet, whereby light incident on the backsheet can be effectively reflected and reused to enhance the power conversion efficiency of the solar cells. Patent Literature 1 discloses a white resin film formed of a resin composition obtained by adding a white pigment such as titanium oxide to a thermoplastic resin such as polyethylene terephthalate, polycarbonate, polymethyl methacrylate, polyacrylate, polyethylene naphthalate or acryl.
Japanese Patent Application Laid-Open No. 2007-35694 (Patent Literature 2; corresponding to EP 1938967 A1) discloses a backsheet for solar cell module with a cured coating film of a curable functional group-containing fluoropolymer paint formed on at least one surface of a water-impermeable sheet. Patent Literature 2 describes that a white pigment such as titanium oxide or a black pigment such as carbon black is desirably incorporated into the curable functional group-containing fluoropolymer paint for making the appearance of a solar cell module beautiful.
Patent Literature 2 exemplifies various kinds of fluoropolymers and describes that among these, a tetrafluoroethylene (TEL) polymer is preferred because the polymer is excellent in dispersibility of a pigment, weather resistance, copolymerizability with a curable functional group-containing monomer and chemical resistance. Example of Patent Literature 2 shows a backsheet obtained by forming a white cured coating film with a white paint containing a curable TEL polymer and titanium oxide on a water-impermeable sheet.
Japanese Patent Application Laid-Open No. 2007-208179 (Patent Literature 3) discloses a plastic film for back surface protecting film of a solar cell having an average reflection rate of at least 70% in a wavelength range of 600 to 1,400 nm. This plastic film for back surface protecting film of the solar cell has high reflection properties in both visible light region and near-infrared light region and exhibits a function of enhancing a photovoltaic power generation efficiency. Patent Literature 3 describes that an inorganic white pigment such as titanium oxide is incorporated into a plastic making up the plastic film for achieving high reflection properties. Patent Literature 3 exemplifies various kinds of thermoplastic resins as the plastic and describes that among these, polyesters are particularly preferred. Example of Patent Literature 3 shows a plastic film formed of a resin composition obtained by adding an inorganic white pigment such as titanium oxide into a polyethylene terephthalate copolymer.
International Publication No. 2008/157159 (Patent Literature 4; corresponding to EP 2158614 A1) discloses a solar cell module equipped with a backsheet comprising a functionalized polyvinylidene fluoride (PVDF) resin composition. The PVDF resin composition forms an outermost layer of the backsheet. Examples 1 and 2 of Patent Literature 4 show laminated films obtained by forming a coating layer of a PVDF resin composition with a coating liquid containing a PVDF resin, polymethyl methacrylate and titanium oxide on a polyethylene terephthalate (PET) film.
International Publication No. 2007/085769 (Patent Literature 5; corresponding to US 2009/0275251 A1) discloses an invention relating to a multi-layer structure comprising a PVDF film and a polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) film, and Examples 1 and 2 thereof show multi-layer films comprising a resin composition layer containing a PVDF resin, titanium oxide and polymethyl methacrylate (PMMA).
International Publication No. 2008/019229 (Patent Literature 6; corresponding to US 2010/0000601 A1) discloses a solar cell module comprising a transparent and glossy material composed of a solid thermoplastic substrate layer and a PVDF outer layer and shows that PMMA may be blended into the PVDF outer layer. This PVDF outer layer is arranged as a front surface layer of the solar cell module and is not a white resin film containing titanium oxide.
As the backsheet for solar cell module, is generally used a single-layer or multi-layer plastic film, a plastic plate, reinforced glass plate, a metal plate, a composite of a plastic film and a metal plate, a composite of a plastic film and a metal foil or the like. As the metal plate, is also used that obtained by forming a synthetic resin coating film on the surface thereof.
As the plastic film, a fluororesin film, a PET film and a composite film thereof are preferred from the viewpoint of satisfying the various properties required of the backsheet for solar cell module. The fluororesin film and the composite film of the fluororesin film and the PET film are more preferred from the viewpoints of light resistance, weather resistance, heat resistance, stain resistance, etc.
As the fluororesin film for the backsheet for solar cell module, is generally a polyvinyl fluoride (PVF) resin film. However, a polyvinylidene fluoride (PVDF) resin film is more suitable for use as the backsheet for solar cell module than the PVF resin film from the viewpoints of weather resistance, stain resistance, heat resistance, etc. When a white resin film formed of a PVDF resin composition obtained by containing titanium oxide into a PVDF resin is used as a backsheet for solar cell module, the appearance of the module can be made beautiful, and in addition, the film is expected to contribute to improvement in power conversion efficiency of solar cells.
By the way, according to the results of researches by the present inventors, it has been proved that when titanium oxide is contained in a PVDF resin, the thermal decomposition temperature of the PVDF resin is greatly lowered. The PVDF resin has a melting point of 177° C. and a thermal decomposition starting temperature of 350° C. When the resin is heated to 350° C. or higher, hydrogen fluoride (HF) gas is generated to decompose the resin. These melting point and thermal decomposition starting temperature are both typical values of the PVDF resin. The fact that the range from the melting point to the thermal decomposition starting temperature is wide indicates that the processing temperature of the PVDF resin is wide. On the other hand, titanium oxide is particularly excellent in color tone and shielding ability (light scattering ability) among inorganic white pigments and can contribute to improvements in the color tone and reflection properties of a white resin film.
In order to form a white resin film from the PVDF resin, it is necessary to blend a relatively large amount of titanium oxide into the PVDF resin. It has been proved that when a PVDF resin composition obtained by causing a relatively large amount of titanium oxide to be contained in the PVDF resin is extruded to form a white resin film (including a sheet), the resultant white resin film is liable to change its color to brown, and the PVDF resin becomes easy to be thermally decomposed. It has been found that when a PVDF resin composition obtained by causing titanium oxide in an amount enough to impart shielding ability and whiteness degree that are suitable for a backsheet for solar cell module to be contained in the PVDF resin is subjected to a thermogravimetric measurement by a thermogravimetric analysis (TGA), a temperature at 10%-weight loss on heating is lowered by all of from about 40° C. to about 45° C. compared with the PVDF resin alone. The temperature at 10%-weight loss on heating of the PVDF resin is typically within a range of from about 382° C. to about 385° C. On the other hand, the temperature at 10%-weight loss on heating of, for example, a PVDF resin composition obtained by causing 30 parts by weight of titanium oxide to be contained in 100 parts by weight of this PVDF resin is lowered to a range of from about 336° C. to about 342° C.
In addition, when a film formed from the PVDF resin composition containing the PVDF resin and titanium oxide is subjected to a heat test in a gear oven heated to a temperature of 230 to 270° C., the film is changed in its color to dark brown after several hours, and bubbling presumed to be traces of having generated gasses by decomposition is also observed. Even when polymethyl methacrylate compatible with the PVDF resin is contained in the resin composition containing the PVDF resin and titanium oxide, lowering of heat resistance and color change by heating which are attributable to titanium oxide cannot be improved. Such defects cannot be solved even when a method of using the PVDF resin composition as a coating liquid to form a coating film is adopted.
Although the PVDF resin film has excellent various properties suitable for use as a backsheet for solar cell module, lowering of heat resistance and appearance is marked when titanium oxide is contained therein, and such a film become poor in durability. Therefore, it has been extremely difficult to obtain a white resin film capable of making the appearance of a solar cell module beautiful, and enhancing the power conversion efficiency of solar cells, and excellent in durability by using the PVDF resin composition obtained by blending titanium oxide into the PVDF resin.